The present invention relates to a monochromator and a spectrometric method for projecting a measured beam (a beam which is measured) on one and the same diffraction grating a plurality of times.
Conventionally, a spectroscope called xe2x80x9cmonochromator,xe2x80x9d has been used as an instrument to measure wavelength characteristics of a measured beam. Particularly, a double monochromator is widely used to allow keeping a high resolution or a wide dynamic range by incidence of a beam into one or more diffraction gratings a plurality of times.
For example, a variety of monochromators have been Japanese Patent Laid-Open No. 8-145795. FIGS. 4 and 5 are perspective side views showing the configuration of representative monochromators disclosed in the publication.
The monochromator shown in FIG. 4 converts a beam emitted from an optical fiber 100 to a parallel beam by a collimator 102 followed by diffracting this parallel beam by a plane diffraction grating 104. The diffracted beam is reflected by a plane mirror 106 having a reflecting surface perpendicular to the beam path, diffracted by the plane diffraction grating 104 followed by condensing by the collimator 102, and finally passes through a slit 108. A monochromator shown in FIG. 4 allows increasing the resolution of the wavelength xcex of the diffracted beam passing through the exit slit 108, because the measured beam is diffracted twice in the identical plane diffraction grating 104.
In comparison with the structure shown in FIG. 4, the monochromator shown in FIG. 5 has a structure comprising an intermediate slit 110 and two mirrors 112 and 114. In the monochromator shown in the FIG. 5, the diffracted beam returned by reflection by the collimator 102 is reflected 90xc2x0 by one mirror 112, passed through the intermediate slit 110 located in the condensing position of the diffracted beam, and reflected 90xc2x0 by the other mirror 114 to return one more time through an optical system comprising the collimator 102, the plane diffraction grating 104, the plane mirror 106. Thus, the monochromator shown in FIG. 5 allows the dynamic range of the beam to widen by passing through the intermediate slit 110 and the exit slit 108.
As other conventional examples of the monochromator, those disclosed in U.S. Pat. Nos. 3,069,966 and 4,025,196 have been known.
Meanwhile, the conventional monochromator shown in FIG. 4 requires to locate both the optical fiber 100 used for incidence of the measured beam and the exit slit 108 in around the position of the focal point of the collimator 102 to make the structure around the focus position complex to disturb such work as assembling. Furthermore, the conventional monochromator shown in FIG. 5 requires to locate around the two mirrors 112 and 114 and the intermediate slit 110 in addition to the optical fiber 100 and the exit slit 108 around the position of the focal point of the collimator 102 to make the structure around the focus position more complex to disturb further such work as assembling.
The present invention created in consideration of such problems; the object is to provide a monochromator and a spectrometric method to allow such work as assembling by simplify the structure of the part where a measured beam is incoming and outgoing.
A monochromator of the present invention comprises a plane diffraction grating; a first collimator and a second collimator that are located in parallel to rulings of the plane diffraction grating; a first reflecting member that has at least two reflecting surfaces and returns a diffracted beam emitted from the plane diffraction grating so that an incident beam and an outgoing beam of the diffracted beam separate from each other along the rulings; and an exit slit located near a position of a focal point of the second collimator. By having the first reflecting member to separate and return an incident beam and an outgoing beam and the first and the second collimators for respective two separated rays, the exit slit may be located in the position of the focal point of the second collimator and other optical members may be located in the position of the focal point of the first collimator with a distance from each other. Therefore, the structures around respective positions of focus are simplified to improve such work as assembling.
More specifically, it is preferable that the incident member receiving the measured beam is located around the position of the focal point of the first collimator. Separating the incident member from exit slit with a distance simplifies respective fitting portions, increases a freedom of designing, and makes such work as mounting easy. Besides, improvement of resolution may become possible on the basis of that the identical plane diffraction grating carries out diffraction a plurality of times.
Alternatively, it is preferable that the exit slit and the incident member that receives the measured beam are located around the position of the focal point of the first collimator and that the intermediate slit and the second reflecting member, which is located in both outsides of the intermediate slit to reflect the emitted beam from the second collimator toward the second collimator, are located around the position of the focal point of the second collimator. Structures around the exit slit may be separated from the intermediate slit and the second reflecting member with a distance. Therefore, in comparison with that all these are located around the exit slit as conventional examples, respective parts maybe arranged more freely to allow freedom of designing and easy mounting work. Further, the dynamic range of the beam that passes through the exit slit may be widened by allowing to pass the measured beam through the intermediate slit in reflection of the measured beam by the second reflecting member.
Particularly, it is preferable that the direction of the intermediate slit in parallel to the rulings and that the two reflecting surfaces of the second reflecting member are located along the direction in which the beam emitted from the second collimator is swayed, when the plane diffraction grating is rotated about an axis which is parallel to the rulings of the grating. By such arrangement, an additive dispersion state may be realized to increase furthermore angular dispersion within the width of wavelength of the incident beam on the plane diffraction grating and also an increase in resolution becomes possible.
Alternatively, it is preferable that the intermediate slit is located in a direction that is perpendicular to the rulings and that the second reflecting member is located in a direction along the rulings. By such arrangement, a differential dispersion may be realized to reduce the angular dispersion within the width of wavelength of the incident beam on the plane diffraction grating. Under the condition of differential dispersion, the width of the exit slit need not change, even if the wavelength of the measured beam is changed, to make simplifying the structure possible.
The above described first reflecting member is preferable to emit the outgoing beam in a direction that is almost 180xc2x0 opposite the direction of the incident beam. The exit slit may be easily disposed separately with a distance from other parts easily by locating the two collimators corresponding to these positions with the distance, because almost parallel reflected beam separated from the incident beam with the distance is returned.
Further, a spectrometric method of the present invention comprises the steps of: diffracting a measured beam converted into a parallel beam by a first collimator, by a plane diffraction grating; returning the diffracted beam so that the diffracted beam after the return is separated from and is almost parallel to that before the return along rulings of the plane diffraction grating; diffracting the diffracted beam again by the plane diffraction grating; condensing the diffracted beam by a second collimator; and allowing the diffracted beam to pass through an exit slit located in a position where the diffracted beam is condensed. The diffracted beam in the plane diffraction grating is returned to a separated position along the rulings and projected into the plane diffraction grating again in order to separate the focus positions of the two collimators, which have been installed to correspond to respective incident beam and outgoing beams, with a distance. Therefore, resolution may be improved and workability is also improved by simplifying the structure.
A spectrometric methods of the present invention comprises the steps of: diffracting a measured beam converted into a parallel beam by a first collimator, by a plane diffraction grating; returning the diffracted beam by a first reflecting member so that the diffracted beam after the return is separated from and is almost parallel to that before the return along rulings of the plane diffraction grating; diffracting the diffracted beam again by the plane diffraction grating; condensing the diffracted beam by a second collimator; returning the diffracted beam to almost the same beam path through an intermediate slit and a second reflecting member that are located in a position where the diffracted beam is condensed; and allowing the diffracted beam to pass through an exit slit located in the position where the diffracted beam is condensed by the first collimator. By such arrangement, the structure around the exit slit may be separated from the intermediate slit and the second reflecting member with a distance. Thus, respective parts may be arranged more freely to allow freedom of designing and easy mounting work. Further, the dynamic range of the beam that passes through the exit slit may be widen by allowing the measured beam to pass through the intermediate slit in reflection of the measured beam by the second reflecting member.